(1) Technical Field of the Invention
This invention relates generally to non-destructive testing methods and devices which are employed for detecting the presence and measuring the size of defects in metal parts. More particularly, this invention relates to methods and devices which employ ultrasonic energy for detecting and measuring cracks in, for example, the wall of a metal pipe.
While not limited thereto in its utility, the present invention is particularly well-suited to use in the nuclear industry. The safety of nuclear power facilities has become a matter of increasing public concern. A principal focus of this concern has been the integrity of the conduit systems employed to circulate coolant in nuclear power plants. Non-destructive testing to detect and examine cracks, which result from intergrannular stress corrosion or fatigue for example, originating at the inner diameter of clad or unclad piping of a nuclear system must be performed from the outside surface of the pipes and when the system is in service. Ultrasonic testing has proven to be the only viable approach to detecting cracks in such pipes while the system is in service.
Although ultrasonic testing has generally proven to be economically acceptable, previously employed ultrasonic techniques have been characterized by difficulty in accurately identifying false readings. Such difficulty often prevents the obtaining of reliable inspection results. The complex multiple refraction and back-scattering phenomena which is produced when a beam of ultrasonic energy is coupled into the wall of a metal pipe generally results in echos received at sensor (receiver) transducers having low signal-to-noise ratios which make interpretation of data very problematical and frequently less than reliable in terms of accurately identifying defects. The unfavorable signal-to-noise ratios make the obtaining of reliable information which could be employed for determining the dimensions of the located defects even more problematical. Accordingly, it is a principal aim of the present invention to provide a new and improved method and device for ultrasonic testing which results in a favorable signal-to-noise ratio of the test data.
(2) Prior Art
U.S. Pat. No. 4,435,984 discloses an ultrasonic multiple beam technique for detecting cracks in bi-metallic coarse grain materials. The patented method employs multiple beams, bands and pulses in addition to pulse shaping and beam forming, spectral and directional averaging and spacial filtering and pattern recognition. The method of U.S. Pat. No. 4,435,984 employs a pair of spaced transducers mounted at different angular orientations relative to a pipe to be inspected. In normal operation short shear wave pulses are serially propogated in the test object by the spaced transducers and an effort is made to analyze the echos received at the transducers to determine characteristic of any defects. A manual confirmation mode contemplated by the patentee included transmitting both longitudinal waves and shear waves toward a suspected defect.